CONTINUOUSWAVE

I've had [ a 2006 170 MONTAUK] boat for one year. [The perfomance] feels sluggish.

My goal is to gain stop end boat speed. A top-end boat speed of 40-MPH would be nice. To [be able to at an engine speed of 4,000-RPM and get a boat speed between 25 and 30-MPH] would be nice.

A typical load [remember this because it is mentioned several times] is two adults whose combined weight is less than 300-lbs, a fuel load of 15-gallons to 20-gallons of gasoline, and minimal gear. There are two batteries in console.

A heavy load is myself and two other fisherman, more gear, and a full 20-gallon rear livewell.

The 2006 170 [Montauk] boat has PETIT VIVID bottom paint. [In the future there may be] a trolling motor and two Group-27 batteries added to the 2006 170 MONTAUK.

The 2006 Mercury 90 FOURSTROKE [is actually made by Yamaha] and has 130 hours. Engine mounting height is one-hole up; there are five holes.

The current Prop is a [Mercury part numer 48-16988 [otherwise known as a] VENGEANCE three-blade 18-pitch.

The top boat speed over ground per GNSS in flat calm, no wind conditions with the engine trimmed up and with the "typical load] [mentoned above] is 37.5-MPH with an engine speed of 5400- to 5500-RPM.

With a wind of 10-[knots--note wind speeds are usually specified in knots], and the engine trimmed up, and with the same "typical load" [mentioned above] the boat speed is 34-MPH and the engine speed is between 5400 to 5500-RPM.

The average cruising boat speed is 22 to 23-MPH with an engine speed of 4000-RPM.

Q1: Should I try the Laser II 20-pitch propeller?

Q2: Should I raise the engine mounting height [to two-holes up]?

ANOTHER TOPIC:

I also get some grinding occasionally when pulling the throttle back towards neutral.

Q3: should I put in a FLO-TORQ III hub?

That is a popular combo. I would assume people will chime in on their setup. Is the vengeance the right prop? Is that a 4blade? If so then you probably want to go with the laser. I would go up 2 holes and test. How trimmed is trimmed up? At WOT you should trim till it starts to slip then tap it down until it grabs. 4 strokes like to be mounted a bit higher in my opinion as well.

bmolt12 wrote:Q3: should I put in a FLO-TORQ III hub?

Mercury gear cases are notorious for shifting like a 1950's truck. If you want to reduce the CLUNK, the FLO-TORQ III hub might help.

Tom Clark and I collaborated to write the definitive article about FLO-TORQ hubs. See:

Flo-Torq Adapters
http://continuouswave.com/ubb/Forum4/HTML/005155.html

The comment about shifting like a 1950's truck comes from Mercury's head of four-stroke-power-cycle outboard engineering. See

http://continuouswave.com/forum/viewtopic.php?t=2198#p12735

The published performance report of the 2007 Montauk 170 with a Mercury ELPT FourStroke 90hp with a 13-1/4 x 20 Laser II prop lists a wide open throttle (WOT) speed of 42.8-MPH at engine speed 5,900-RPM under the following conditions:

Dry weight of 1924
Fuel - 6 gallons
Test Gear - 0 lbs
Personel - 330
Test weight of 2,291

cf: http://bostonwhaler.com.s3.amazonaws.com/resources/170-Montauk-Performance-2010.pdf

Taking into consideration that the engine on your boat may not be identical in output, the propeller selection, and you have the following additional items on board as a typical load:

Bottom paint (loss of 2 to 3-MPH)
Second battery
More fuel
More gear

One can infer that your boat when "typically loaded" is performing close to the factory performance report.

If you did want a minor gain, you can raise the engine one hole which would result in a rpm gain at WOT of about 150-RPM.

Before considering a propeller change, I would verify the engine model, lower unit gear ratio, and WOT engine speed RPM range.

If the engine you have is the same as in the performance report, a change to a Laser II in the size listed would be appropriate.